Typical examples of automation technology equipment are position-measuring devices and the subsequent electronics in conjunction with which they are used. The subsequent electronics may be, for example, numerical machine tool controls or complex plant control systems.
Position-measuring devices are used, for example, to determine, in closed-loop drive mechanisms, actual position values which are needed by subsequent electronics, such as a numerical control system, to calculate setpoints for control circuits used to control the drive mechanism (e.g., the feed rate of a tool or workpiece). If the position-measuring devices are in the form of rotary encoders or angle-measuring devices, then these are directly or indirectly coupled, for example, to the shaft of a motor for this purpose. Length-measuring devices measure, for example, linear movements between a machine bed and a machine part that is positionable relative to the machine bed, such as, for example, a movable tool carriage.
Today, absolute position-measuring devices are preferably used. Such devices generate absolute measurement values, which are transmitted from the position-measuring device to the subsequent electronics via digital, mostly serial data interfaces. The measurement values are mostly position values (angular values or linear positions), but there are also known position-measuring devices which deliver velocity or acceleration values; i.e., measurement values which indicate changes in positions over time.
Frequently, long distances must be spanned between measuring devices and their subsequent electronics for data transmission purposes, especially in the case of large plants or plants which are distributed over a large area. As a result, the signal quality of the digital signals deteriorates over the line length, making it more difficult at the receiver end to correctly read arriving data streams.
This can be counteracted by using high-quality data cables which, however, significantly increases the cost of the overall system.
Another way of achieving reliable data transmission in spite of long cable lengths is by reducing the data transfer rate. This, however, adversely affects the dynamic performance of control circuits which, for example, require position values from position-measuring devices as actual position values to be able to calculate new desired position values.
International Patent Application WO 03/067804 A1 proposes to determine a bit error rate at the receiver end, and to take actions as a function thereof, such as to request a repetition of the transmission or to reduce the length of data packets to be transmitted. These actions also have negative effects on the operation of dynamic control circuits.